1. Field of the Invention
The present invention relates to a CVD apparatus and its purging method, and more particularly, to the composition of a CVD apparatus that is able to shorten the time required for purging treatment after performing maintenance, and its purging method.
In addition, the present invention relates to a moisture monitoring apparatus that measures the moisture contained in corrosive gas in the process when performing epitaxial growth and so forth using corrosive gas on a silicon wafer arranged in, for example, a reactor, a semiconductor production apparatus equipped with said moisture monitoring apparatus, and a method for judging the maintenance times of semiconductor production apparatuses.
2. Description of the Related Art
CVD apparatuses are apparatuses used to grow a semiconductor film on a wafer by chemically reacting a semiconductor gas introduced into a reactor on the wafer. However, since it is not possible in principle to react all material gases on the wafer, by-products end up adhering to various locations on the inside walls of the reactor. These by-products have an effect during film growth in the form of particles and so forth. Since they hinder the formation of a high-quality film, it is necessary to performing cleaning work (maintenance) on the inside walls of the reactor.
In a CVD apparatus for growing thick films, for example, maintenance is required on the order of once every 3-4 days. However, since the apparatus is opened to the atmosphere and is cleaned with alcohol and so forth during maintenance, a large amount of air is taken into the reactor resulting in the adherence of moisture on the inside walls of the reactor.
If moisture is present in the atmosphere during growth of semiconductor films, it reacts with the semiconductor material gas resulting in the formation of metal impurities or the formation of particles that worsen the film quality. Consequently, following maintenance, it is necessary to purge the inside of the apparatus with high-purity nitrogen or other insert gas before growing films to lower the moisture concentration to an extent that does not have a detrimental effect on film quality.
However, since the inside of CVD apparatuses have an extremely complex shape, and the adsorption strength of water molecules is extremely high, considerable time is required for removing moisture after maintenance, which in turn has a significant effect on the operation rate of the apparatus.
In the past, various contrivances have been made to shorten the time required for apparatus maintenance, including purging, examples of which include vacuum purging, heated purging (baking), the use of hydrogen or a combination of these. However, since the conditions and combination of vacuum purging and baking are determined based on experience, it has been difficult to optimize the purging method.
In addition, the completion of purging is judged based on an evaluation of the quality of a film that is actually grown following a certain degree of purging. Consequently, material gas and time were wasted on growth until a film having product level quality was obtained. This is referred to as wasted epitaxial growth. Since the amount of time required for purging varies according to the usage history of the CVD apparatus and the degree to which maintenance has been performed, there were cases in which the number of cycles of wasted epitaxial growth increased considerably.
In recent years, epitaxial wafers, in which a single crystal silicon thin film (epitaxial layer) is vapor deposited at a prescribed impurity concentration on a silicon wafer having extremely low resistivity, are produced with an epitaxial crystal growth apparatus for use as silicon wafers for MOS devices. This apparatus performs epitaxial growth on a wafer by allowing a corrosive source gas to flow into a chamber in which a silicon wafer has been arranged. Furthermore, in this apparatus, etching of polysilicon adhered inside the chamber is also performed by a corrosive gas in the form of hydrogen chloride gas.
In addition, various CVD apparatuses that form a thin film on a wafer using corrosive gas, or etching devices for performing patterning, are used in LSI and other semiconductor production processes.
Although these semiconductor production apparatuses use corrosive gases such as ultra-high-purity hydrogen chloride gas and ammonia gas, if even a slight amount of moisture is present in this gas, there is increased susceptibility to the occurrence of corrosion of metal parts used in the apparatus (such as the inside of the process chamber, gas supply system and gas exhaust system), which is harmful because it causes contamination by metals (heavy metals) produced from these metal parts. In addition, moisture taken into the chamber reacts with by-products adhering to the chamber inside walls and exhaust line, which may also be the cause of particle formation. Consequently, although various countermeasures are employed to reduce moisture inside the process chamber, it is difficult to completely remove all moisture. It is therefore necessary to periodically perform apparatus maintenance, namely opening up the process chamber and cleaning the members inside (quartz jigs, etc.). In the past, for example, maintenance times were judged based on the cumulative number of wafers processed in the case of single-wafer CVD apparatuses.
However, the above conventional method of judging maintenance times still has the problems indicated below. Namely, the amount of moisture actually introduced into the chamber each time maintenance is performed varies depending on the contents of work performed and the amount of time the chamber is opened during maintenance. Thus, in the case of judging maintenance times based on the cumulative number of wafers processed as has been done in the past, maintenance is performed for every fixed number of processing cycles regardless of the amount of moisture actually introduced into the chamber, and there was no guarantee that maintenance is performed at suitable times. For example, in the case an amount of moisture was introduced during the previous round of maintenance that is greater than the expected amount, there was the risk of high film quality not being obtained if processing is not performed until the prescribed cumulative number of wafers processed. In addition, in the case the amount of moisture introduced during the previous round of maintenance is comparatively low, maintenance ends up being performed earlier than the time when maintenance is actually required, leading to an excessively high number of maintenance cycles and decreased throughput.
In addition, it is also required to quantitatively analyze with high sensitivity the moisture contained in corrosive gas inside the chamber in terms of reducing the moisture in the process chamber.
Known examples of moisture meters for measuring the moisture in a gas include the crystal oscillator method in which changes in the frequency of a crystal oscillator are measured, and the electrostatic capacitance method in which changes in electrostatic capacitance are measured by adsorbing moisture in a gas. However, since these moisture meters require direct contact with the gas, measurement was unable to be performed in the case of corrosive gases due to the corrosive nature of these gases.
Therefore, a laser moisture meter has been proposed in recent years, such as that described in Japanese Unexamined Patent Application, First Publication No. Hei 5-99845 and Japanese Unexamined Patent Application, First Publication No. Hei 11-183366, that uses infrared absorption spectrometry to measure trace amounts of impurities contained in gases using laser light. This laser moisture meter detects impurities such as moisture based on the intensity of the absorption wavelength by analyzing transmitted laser light when laser light having a prescribed wavelength is directed at a measurement cell while introducing corrosive gas into the measurement cell. Thus, there is no need to adsorb the corrosive gas and measurements can be performed quickly and with high sensitivity.
However, measurement means using the above moisture meter of the background art still have the problems indicated below. Namely, although a portion of the corrosive gas is introduced into the above moisture meter after passing through a sampling pipe after being heated inside a chamber, reaction by-products end up adhering and accumulating on the inside walls of the sampling pipe that leads to the moisture meter, resulting in the risk of obstruction of the sampling pipe. Consequently, it was difficult to use this moisture meter for constant measurement of moisture in corrosive gases during the process, namely for in situ monitoring.